The operating conditions of turbo-engines, especially aircraft engines, have led to the utilization of numerous titanium or titanium alloy components in such engines. It is important that these components should be subjected to a non-destructive checking capable revealing the various defects from which they may suffer. In particular, they should be examined for possible manufacturing defects such as segregations, inclusions, porosity, etc., transformation defects such as cracks, incrustations, heterogeneity, contaminations, etc., and machining or polishing defects such as work-hardening, local overheating, etc. For this purpose, there is in existence an electro-chemical etching process which is well known in the art as the "blue-etch process".
This electro-chemical etching process consists, generally, in carrying out the following operations on the component to be checked:
1. Conventional degreasing by immersion in an alkaline bath; PA0 2. Rinsing with cold water in a tank of running water, or by sprinkling; PA0 3. Possible removal of a work-hardened layer, about 5 microns, by fluo nitric etching; PA0 4. Rinsing with cold water in a tank of running water; PA0 5. Chemical activation by immersion in an acid salt bath for etching with a macrographic effect; PA0 6. Rinsing with cold water in a tank of running water; PA0 7. Anodic oxidation in a trisodium phosphate bath, with the component to be checked being in the anode position; PA0 8. Rinsing with cold water in a tank of running water; PA0 9. Development by partial etching in a nitrohydrofluoric bath; PA0 10. Rinsing with cold water as quickly and thoroughly as possible, followed by drying of the component; and PA0 11. Reading the defects revealed, on the basis of shapes and colours (white, blut, grey-blue) which are peculiar to them.
However, this process does have some drawbacks. In particular, the existing products which are generally used for the chemical activation and are available in the trade are relatively costly, difficult to use, and deteriorate rapidly.
More particularly, when these commercially available products are used in conditions enabling between 0.8 and 1.3 .mu.m of material to be removed in a 90- second immersion, i.e. with a concentration of 250 g/l, it is a drawback that between steps 5 and 6 of the process described above, i.e. between removal of the component from the chemical activation bath and immersion in the rinsing tank, sufficient time elapses for the etching reagent which is carried with the component and its support to continue acting, causing drainage marks. These marks will prevent, or at least limit, the reading of defects drawn in the same direction, such as fibrillation for example, their intensity being a function of the geometry of the component, the type of supporting structure used, and the aggressivity and the temperature of the bath. Moreover, the operative life of the bath is short at the prescribed rate, and there is no possibility of adding salt to compensate for exhaustion since it is close to saturation. In addition, temperature control is compulsory as the etching rate is multiplied by a factor of 1.5 or 2 for a 10.degree. C. rise of bath temperature. Finally, on immersion of the components in the bath, the etching rate does not reach its maximum immediately, but only after a period termed the "depassivation period", which may last from 10 to 20 seconds. This depassivation time is not negligible in comparison with the immersion time of the components, i.e. about 1.5 minutes, and leads to systematic errors.
To overcome these various drawbacks, thought has been given to using a lower bath concentration of 120 to 150 g/1, with very frequent recharges of +30 to +60 g/1 intended to maintain a certain degree of efficiency at the operating temperature. Unfortunately, while drainage marks are limited in this way and do not hinder the fault reading, the life of the bath is rather short and, in particular, quality assurance is very chancy. Indeed, the reaction rate is much too variable and prohibits the automation of the process, the thickness removed is small, depassivation time is too long, and the bath recharges are not very effective.